The present invention relates generally to a focal plane shutter for a camera, and more particularly to a focal plane shutter comprising a group of blades divided into a plurality of blade units operable to open and close a shutter opening.
The blade mechanism of a conventional focal plane shutter for cameras comprises a group of blades divided into a plurality of blade units. Each blade unit comprises a leading blade and a plurality of trailing blades in superposed relation to the leading blade. The blades of each blade unit are pivotally connected to one end of arm members by a first set of pivot pins, and the arm members are supported at the other end to one side of a shutter base plate by a second set of pivot pins at positions from top ends to intermediate portions of the arm members (see Japanese Patent Publication No. S54-191753). This type of conventional focal plane shutter operates efficiently and is suitable for fast operation since all of the blades are directly or indirectly supported by the arm members.
In order to meet the demand for a faster exposure time following the improvement of film sensitivity, it has been proposed to reduce the overall weight of the blade unit by reducing the size of the pivot pins. However, when the size of the pivot pins is reduced for the purpose of reducing the overall weight of the blade unit, the durability of the blade unit is compromised due to the extreme pressure applied on each of the pivot pins during an exposure operation.
The pivot pins connecting the trailing blades to the arm members have been formed with a through-hole along the axis thereof so as to have a hollow configuration, thereby minimizing the weight of the overall blade unit. This has been made possible since the pivot pins supporting each of the trailing blades are either covered by a superposed trailing blade or not located at the shutter opening. Thus, such pivot pins do not give rise to leakage of light and can thereby be made hollow to reduce the weight of the blade unit.
However, the pivot pins connecting the leading blade to the arm members have the largest inertia mass and are located in the shutter opening when the shutter opening is covered. Thus if the pivot pins connecting the leading blade to the arm members are provided with a through-hole to reduce the overall weight of the blade unit, leakage of light results causing the film to be exposed. Accordingly, it has been difficult to reduce the overall weight of the blade units of conventional focal plane shutters.